Import the "signal-h" module from gnulib

Import the "signal-h" module from gnulib

This gives us a signal.h replacement that makes sure the sighandler_t
typedef (a GNU extension) is always available.  A follow up patch will
make use of this.

gdb/ChangeLog:
2015-08-27  Pedro Alves  <palves@redhat.com>

	* gnulib/update-gnulib.sh (IMPORTED_GNULIB_MODULES): Add signal-h.
	* gnulib/aclocal.m4: Renegerate.
	* gnulib/config.in: Renegerate.
	* gnulib/configure: Renegerate.
	* gnulib/import/Makefile.am: Update.
	* gnulib/import/Makefile.in: Regenerate.
	* gnulib/import/m4/gnulib-cache.m4: Update.
	* gnulib/import/m4/gnulib-comp.m4: Update.
	* gnulib/import/m4/signal_h.m4: New file.
	* gnulib/import/signal.in.h: New file.